The present invention relates to a measuring circuit for a differential-choke transmitter, i.e. to a measuring circuit which generates a measurement voltage for measurement of the displacement of an adjusting rod of a displacement transmitter. The invention also relates to a method of using the measuring circuit.
An evaluating circuit which can be used in combination with various conventional displacement transmitters is described in an article entitled "Using the NE 5521 Signal Conditioner in Multi-faceted Applications", by Z. Rahim in Valvo-Datenbuch 1988, Professional Integrated Analog and Special Circuits [Professionelle integrierte Analog- und Spezialschaltungen], Dr. Alfred Huttig Verlag, Heidelberg, pages 818 and 819. This is a circuit with a measuring principle such as has been used many times. Namely, a sinusoidal voltage is applied to a transmitter, e.g. a differential-choke displacement transmitter, and the induction voltage is evaluated, which is effected by phase-coupled rectification and smoothing of the rectified induced sine signal. A problem in circuits with this known principle consists in that considerable expenditure must be made on circuits which smoothing the measurement signal so that the latter changes in proportion to the adjustment path of the adjusting rod of the transmitter without excessive residual ripple.
An evaluating circuit which works according to another principle for application with a semi-differential short-circuit ring displacement transmitter is described in EP 88 903 834. This is a circuit in which an alternating voltage of constant amplitude and fixed frequency is applied to a first output terminal and an alternating voltage of identical frequency, but variable amplitude, is applied to the other output terminal. The adjustable amplitude is varied until the voltage tapped at the center tap of the transmitter is zero. The value of the variable amplitude as presented in this balancing condition is a direct measurement for the path adjusted at the transmitter. The value of the amplitude is in turn directly proportional to the value of a control voltage by which the amplitude is adjusted. This means that the control voltage is also directly proportional to the adjustment path. Since the control voltage is a d.c. voltage, the measurement voltage has no ripple, specifically without the use of expensive smoothing measures.
As is the case with a semi-differential short-circuit ring displacement transmitter, a differential-choke displacement transmitter likewise has two end terminals and a middle terminal. However, if a differential-choke displacement transmitter were connected to the evaluating circuit known from the aforementioned European publication in the same way as the semi-differential short-circuit ring displacement transmitter, the measurement voltage would change in a sharply nonlinear manner with the adjustment path of the adjusting rod of the transmitter. If a measurement signal which changes in a linear manner with the adjustment path is also to be obtained when a differential-choke displacement transmitter is connected, the connection must be effected via a resistance network with the aid of a d.c. voltage source. The German Patent Application DE 39 27 833, which does not have a prior publication date, describes how such a connection can be effected. A disadvantage in connecting the differential-choke displacement transmitter with the aid of a d.c. voltage source consists in that a d.c. voltage is necessary at the input terminal, which causes parasitic capacitances with ground can to generate erroneous characteristic lines. The relationship between the adjustment path and measured signal is accordingly somewhat nonlinear, although precisely a linear relationship should be caused with the connection via the network with the aid of the d.c. voltage. The parasitic capacitances to ground would not have any effect on the characteristic line configuration if there were no d.c. voltage at the input terminal in the case of balancing.
With respect to the problems described here there has always been a desire to have available an evaluating circuit for a differential-choke displacement transmitter providing a measurement signal without residual ripple, which changes in proportion to the adjustment path of the adjusting rod of such a transmitter without producing erroneous characteristic lines. Moreover, how such a differential-choke displacement transmitter can be used in combination with such a circuit has been problematic.